Part 6 focussed on the patch antenna being used as the feed for the 80cm dish. In this part I describe the efforts I have made to improve the 2.4GHz transmit amplifier, originally described in part 1 and part 4. In particular I wanted to see if the power output could be increased.
Power Amplifier ModificationsMy first problem is that I cannot measure the actual RF output, as I do not have a microwave power meter. However, I can measure the DC current drawn from the power supply, which gives an indication of output power. I found some data on the internet relating DC supply current of the EP-AB003 amplifier vs supply current (www.ra3apw.ru/qo-100/qo-100-stc7031_appn2/). Another crude way of evaluating improvements in uplink power is to monitor the downlink from the satellite.
Some radio amateurs claim that they have increased the gain and/or output power of the EP-AB003 amplifier by bypassing the Tx/Rx switching devices and reducing the voltage drop on the power PCB tracks (traces). I do not need the amplifier to switch between transmit and receive since I previously modified it to be stuck permanently in transmit mode. The photos show the mods I carried out. Bypassing the Tx/Rx switching device on the input side was very difficult to do and this put me off doing a similar mod on the output switching device.
|Bypassing the Input Tx/Rx Switch with a Wire Link|
|The thick red wire reduces the voltage drop in the power rail.|
Unfortunately the modifications produced no measurable improvement in the amplifier performance.
Increasing the Gain of the Transmit ChainThe next question to answer was whether there was enough gain in the transmit chain to drive the EB-AB003 power amplifier to maximum output. The existing setup comprised of the ADALM Pluto SDR, followed by two cascaded SPF5189Z wideband amplifiers, a 2.4GHz bandpass filter, and finally the power amplifier. I decided against adding another SPF5189Z amplifier in the chain as this would result in the third amplifier being overdriven. So for the additional amplifier I went for a small 2.4GHz power amplifier, based on a RF2126 chip, capable of delivering up to 1W. The amplifier (shown in the photo) was quite cheap, but probably slightly over-engineered with its large integral heatsink. Certainly, with the 1W amplifier following the two wideband amplifiers I was able to overdrive the main power amplifier. Unfortunately, during the experiments I may have damaged the 2.4GHz bandpass filter, as it became a lot more lossy, so I removed it. With the power amplifier being driven harder there was no noticeable increase in the signal strength of the signal coming back from the satellite. I concluded that having three amplifiers between the Pluto SDR and the power amplifier was unnecessary, and probably undesirable from a signal quality point of view. I settled on just having the one SPF5189Z wideband amplifier followed by the 1W 2.4GHz amplifier, feeding directly into the main power amplifier. The 1W amplifier offers a lot more headroom than the 2nd wideband amplifier (the EDUP power amplifier typically needs several hundred milliwatts of drive to get full output) so should help prevent compression and distortion. For the moment I am not using a bandpass filter. I feel a little more comfortable with this now one of the wideband amplifiers has been replaced with a 2.4GHz amplifier with reduced bandwidth.This setup gives no less signal back from the satellite then when the power amplifier was being driven a lot harder.
The new 2.4GHz amplifier consumes a lot more current than the SPF5189Z, therefore I have had to upgrade the heatsinking on the 5V linear regulator.
MeasurementsWith maximum drive from the Pluto SDR, the amplifier system draws 1.655 A from a 12V supply. At this drive level the 1W amplifier draws 300mA. Allowing 75mA for the SPF5189Z amplifier, this implies the EB-AB003 power amplifier is drawing 1.28 A. I reckon this corresponds with an RF output power of around 2.5 Watts.
ConclusionsThe modifications have not resulted in any measurable increase in RF output power. So has this all been a waste of time? I do not think so.
- Sometimes it is worth recording the things that do not work well as well as the successes. This is how we learn!
- I am now satisfied that I am getting as much power out of the EDUP EB-AB003 as is practically possible without overdriving it.
Disclaimer: This is my personal blog. Views expressed in my posts are my own and not of my employer. The information provided comes with no warranty. I cannot be held responsible for the content of external websites. Any practical work you undertake is done at your own risk. Please make health and safety your number one priority.